Dripping is a phenomenon commonly occurring due to temperature difference of the inside atmosphere and the outside atmosphere of an “enclosed body” and localized cooling at the interface. The atmosphere within greenhouses surrounded by agricultural film is saturated with water vapor which evaporates from the soil or from the plants, because the surface tension of water is very high (about 72 mN/m) and the surface energy of greenhouse films, which usually made from polyethylene (PE), ethylene-vinyl acetate (EVA) or polyvinylchloride (PVC), is low, the water vapor condenses dropwise on the inner surface of the cold film to cause water dripping. Water droplets on the film greatly reduce the incident sunlight due to irregular reflection and the droplets that fall on the plants can result in physical damage of the crops.
Therefore, to avoid dripping, anti-dripping treatments are used to make the inner surface of greenhouse films hydrophilic in nature or to reduce water surface tension by adding surfactants. Both methods can cause vapor condensed droplets to spread on the surface and drain away, assuring transmission of sunlight into the greenhouse and reducing physical damage to plants.
Currently, most of the anti-dripping films available on the market are incorporated with low molecular weight additives (e.g., nonionic surfactants such as sorbitan esters, polyoxyethylene esters, glycerol esters, and polyglycerol esters) which migrate from the bulk film matrix to the surface of the plastic films. These low molecular weight surfactants on the surface can dissolve in water and reduce water surface tension, so that the “modified water” can spread on the film. Typical anti-dripping additive concentration ranges from 1 to 5 weight percent. Multi-layer films, especially 3-layer films, have been widely used for greenhouse applications. For most of 3-layer films on market, the low molecular weight additives are incorporated into both the inner and middle layers of the film, with identical anti-dripping formulation in these two layers. The middle layer acts as reservoir for anti-dripping additives. However, normally most of these commercially available surfactant additives do not have good affinity with non-polar polyethylene film matrices. These additives can diffuse out of the film quickly and be easily washed away from the film surface by condensed water, leading to reduced anti-dripping service time and the loss of the film's anti-dripping performance in a relatively short time.
Therefore, anti-dripping additives with a longer service time are desired.